In the semiconductor industry, a relatively large silicon wafer (generally on the order of several inches in diameter) is processed to form a multiple number of identical integrated circuits. Once the wafer has been completely processed, it is “diced” apart to form the individual integrated circuits. In most cases, hundreds of identical circuits are formed across the wafer surface. If the performance of the individual circuits is not tested prior to dicing, a “bad” chip may be further processed and packaged, wasting valuable time and money.
Wafer-level testing is well-known in the semiconductor industry and is traditionally used to measure various electrical parameters on each integrated circuit while still in wafer form to verify conformance of the integrated circuit with pre-defined specifications. Beyond the ability to verify conformance to the specifications, wafer-level testing in the integrated circuit industry has the inherent capabilities to identify process problems, provide pass/fail criteria, perform data collection and generate/run specialized tests on the wafer (e.g., customer-specific tests).
The increased use of integrated electronics and optics on a single SOI structure now requires the development of wafer-level testing for both the electronics and optics. This type of wafer-level testing requires electrical inputs/outputs in the form of test pad/points, as well as optical inputs/outputs in the form of couplers, fibers, etc. Commonly-used methods for coupling light into SOI waveguides (such as, for example, inverse nanotapers and three-dimensional tapers) require access to the edge of the chip (or die) to couple into the waveguide structure. U.S. Pat. No. 6,859,587 issued to D. E. Nikonov et al. illustrates one exemplary “edge” coupling method for testing lightwave circuits at the wafer level. In this case, a first optical fiber is coupled to a first “edge” of the lightwave circuit and used to bring a probe/test optical signal into the lightwave circuit. A second optical fiber is coupled to an opposing “edge” of the circuit, and used to collect the output/test optical signal. The need to have access to the “edges” of the circuit is considered to be a severe limitation of this particular wafer-level optical testing method.
U.S. Patent Application Publication 2003/123793 (“Johannessen”), published on Jul. 3, 2003, illustrates an alternative “optical probe” arrangement where testing of a planar lightwave circuit is achieved by removing a top surface portion of the circuit material in selected locations to gain access to a waveguide structure, allowing an optical probe to be brought into direct contact with the waveguide. While this arrangement eliminates the need to perform “edge” contacts, this type arrangement is considered as “destructive testing”, since a portion of the circuit must be removed to perform the testing. Obviously, when performing repeated tests at multiple circuit locations on a wafer, destructive testing is not a preferred choice. Further, it is not clear that this type of optical probe could be used with sub-micron dimensioned optical waveguides, which are finding increased use for single mode communication applications. Moreover, both of these prior art arrangements require the use of index matching fluids between the optical probe and the wafer (raising issues regarding reproducibility of measurements and contamination) and provide only optical testing; a traditional electronic “probe card” is still required to analyze and test the electronics on the wafer.
Thus, a need remains in the prior art for a wafer-level testing methodology that combines optical and electrical testing into a single arrangement.